[google_maps_flutter] Add an interface for test inspection

[stuartmorgan-g]

Adds an inspector interface to the platform interface to use in tests. Platform interface portion of #6116

Part of flutter/flutter#55504

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

[stuartmorgan-g]

This is just the platform_interface subset of #6116 except that I realized I didn't add any tests in the interface package so added google_maps_inspector_platform_test.dart.

[ditman]

Reviewed in #6116. Ship it!